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107 lines
4.5 KiB
107 lines
4.5 KiB
// Copyright (c) 2013-2014 Sandstorm Development Group, Inc. and contributors
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// Licensed under the MIT License:
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//
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// Permission is hereby granted, free of charge, to any person obtaining a copy
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// of this software and associated documentation files (the "Software"), to deal
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// in the Software without restriction, including without limitation the rights
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// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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// copies of the Software, and to permit persons to whom the Software is
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// furnished to do so, subject to the following conditions:
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//
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// The above copyright notice and this permission notice shall be included in
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// all copies or substantial portions of the Software.
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//
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// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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// THE SOFTWARE.
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#include "memory.h"
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#ifndef KJ_REFCOUNT_H_
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#define KJ_REFCOUNT_H_
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#if defined(__GNUC__) && !KJ_HEADER_WARNINGS
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#pragma GCC system_header
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#endif
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namespace kj {
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class Refcounted: private Disposer {
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// Subclass this to create a class that contains a reference count. Then, use
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// `kj::refcounted<T>()` to allocate a new refcounted pointer.
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//
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// Do NOT use this lightly. Refcounting is a crutch. Good designs should strive to make object
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// ownership clear, so that refcounting is not necessary. All that said, reference counting can
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// sometimes simplify code that would otherwise become convoluted with explicit ownership, even
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// when ownership relationships are clear at an abstract level.
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//
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// NOT THREADSAFE: This refcounting implementation assumes that an object's references are
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// manipulated only in one thread, because atomic (thread-safe) refcounting is surprisingly slow.
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//
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// In general, abstract classes should _not_ subclass this. The concrete class at the bottom
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// of the hierarchy should be the one to decide how it implements refcounting. Interfaces should
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// expose only an `addRef()` method that returns `Own<InterfaceType>`. There are two reasons for
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// this rule:
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// 1. Interfaces would need to virtually inherit Refcounted, otherwise two refcounted interfaces
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// could not be inherited by the same subclass. Virtual inheritance is awkward and
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// inefficient.
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// 2. An implementation may decide that it would rather return a copy than a refcount, or use
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// some other strategy.
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//
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// TODO(cleanup): Rethink above. Virtual inheritance is not necessarily that bad. OTOH, a
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// virtual function call for every refcount is sad in its own way. A Ref<T> type to replace
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// Own<T> could also be nice.
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public:
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virtual ~Refcounted() noexcept(false);
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inline bool isShared() const { return refcount > 1; }
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// Check if there are multiple references to this object. This is sometimes useful for deciding
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// whether it's safe to modify the object vs. make a copy.
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private:
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mutable uint refcount = 0;
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// "mutable" because disposeImpl() is const. Bleh.
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void disposeImpl(void* pointer) const override;
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template <typename T>
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static Own<T> addRefInternal(T* object);
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template <typename T>
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friend Own<T> addRef(T& object);
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template <typename T, typename... Params>
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friend Own<T> refcounted(Params&&... params);
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};
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template <typename T, typename... Params>
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inline Own<T> refcounted(Params&&... params) {
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// Allocate a new refcounted instance of T, passing `params` to its constructor. Returns an
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// initial reference to the object. More references can be created with `kj::addRef()`.
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return Refcounted::addRefInternal(new T(kj::fwd<Params>(params)...));
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}
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template <typename T>
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Own<T> addRef(T& object) {
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// Return a new reference to `object`, which must subclass Refcounted and have been allocated
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// using `kj::refcounted<>()`. It is suggested that subclasses implement a non-static addRef()
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// method which wraps this and returns the appropriate type.
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KJ_IREQUIRE(object.Refcounted::refcount > 0, "Object not allocated with kj::refcounted().");
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return Refcounted::addRefInternal(&object);
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}
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template <typename T>
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Own<T> Refcounted::addRefInternal(T* object) {
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Refcounted* refcounted = object;
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++refcounted->refcount;
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return Own<T>(object, *refcounted);
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}
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} // namespace kj
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#endif // KJ_REFCOUNT_H_
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